The basic aim of this proposal is to elucidate the mechanisms which govern the regulation of RNA synthesis in virus infected cells. The small coliphage T3 on infection of E. coli B arrests the macromolecular synthesis of the host and also induces the synthesis of viral specific RNA polymerase. An exploration of this system has revealed the occurrence of a novel protein in T3 infected cells which selectively inhibits E. coli RNA polymerase by blocking initiation of RNA chains. Sigma factor is essential for this inhibition to occur. This inhibitory protein will be purified to homogeneity in an effort to understand the nature of this protein, the stoichiometry, specificity, mechanism and kinetics of this inhibitory reaction. Apart from the production of this inhibitory protein, T3 infection also results in the modification of E. coli RNA polymerase. This modification has been traced to an alteration of the beta' subunit of the polymerase and results in a marked loss of catalytic activity. Further studies will be directed towards understanding the type of modification, the mechanism of modification, and the biological function achieved by such an alteration. The genes responsible for the synthesis of the inhibitory protein and the modification protein will also be identified and their importance in host-virus interactions will be analyzed. An understanding of these processes is not only important from the viewpoint of regulation of viral growth in general, but also for the comprehension of the mechanism of transcription itself.